1. Field of the Invention
The present invention relates to an AGC circuit of an FM receiver.
2. Description of the Related Art
A conventional FM receiver will be described below with reference to FIGS. 3 and 4. FIG. 3 shows a circuit configuration, and FIG. 4 shows input signal levels at which an AGC operation starts to take effect.
First, as shown in FIG. 3, a radio-frequency circuit 21 is composed of a variable attenuation circuit 21a to which an FM broadcast signal is input, and an input tuning circuit 21b, a radio-frequency amplification circuit 21c, and an inter-stage tuning circuit 21d that are provided downstream of the variable attenuation circuit 21a and connected to each other in cascade. The variable attenuation circuit 21a consists of a pin diode etc. The radio-frequency amplification circuit 21c is a variable gain amplification circuit.
A mixing circuit 22 is connected to the output end of the inter-stage tuning circuit 21d. A local oscillation signal for frequency conversion is supplied from an oscillation circuit 23 to the mixing circuit 22. An intermediate frequency tuning circuit 24, a first bandpass filter 25, an intermediate frequency amplification circuit 26, a second bandpass filter 27, and a third bandpass filter 28 are provided in this order downstream of the mixing circuit 22. The intermediate frequency tuning circuit 24 is a transformer (IFT), and the first to third bandpass filters 25, 27, and 28 are ceramic filters. The intermediate frequency tuning circuit 24 and the first to third bandpass filters 25, 27, and 28 have a common pass-band center frequency of 10.7 MHz. On the other hand, whereas the pass-band width of the intermediate frequency tuning circuit 24 is about 500 kHz, that of the first to third bandpass filters 25, 27, and 28 is about 200 kHz.
First and second AGC voltage generation circuits 29 and 30 are provided on the output side of the inter-stage tuning circuit 21d and the intermediate frequency tuning circuit 24, respectively. The AGC voltage generation circuits 29 and 30 output respective AGC voltages, which are both input to a superimposing circuit 31. The superimposing circuit 31 superimposes the two AGC voltages on each other. A higher one of the two AGC voltages are input to the variable attenuation circuit 21a and the radio-frequency amplification circuit 21c. 
The frequency characteristic of the first AGC voltage that is output from the first AGC voltage generation circuit 29 mainly depends on the frequency characteristic of the inter-stage tuning circuit 21d, and the frequency characteristic of the second AGC voltage that is output from the second AGC voltage generation circuit 30 mainly depends on the frequency characteristic of the intermediate frequency tuning circuit 24. Whereas the frequency characteristic of the inter-stage tuning circuit 21d is broad, that of the intermediate frequency tuning circuit 24 is sharp.
Incidentally, FM broadcast signals have frequency intervals of 100 kHz. In an area where a plurality of broadcasting stations that output strong-electric-field signals exist close to each other, an interference signal occurs in the radio-frequency circuit 21 (in particular, the radio-frequency amplification circuit 21c) at a frequency that is separated from the frequency of a broadcast signal of a desired broadcasting station by about 100 kHz. To reduce the influence of such an interference signal, the level of a generated interference signal is suppressed by decreasing the gain of the radio-frequency circuit 21 by generating an AGC voltage using the interference signal. To this end, in a range of the frequency of a broadcast signal to be received plus/minus several hundreds of kilohertz (e.g., 500 kHz), with regard to AGC voltages obtained from an input signal, the second AGC voltage is set higher the first AGC voltage. The latter is set higher than the former for a signal whose frequency is out of the above range.
More specifically, as shown in FIG. 4, in the range of the frequency Fd of a broadcast signal to be received plus/minus 500 kHz, an input signal level N at which an AGC operation of the second AGC voltage starts is set lower than an input signal level W at which an AGC operation of the first AGC voltage starts. On the contrary, in the ranges that are distant from the frequency Fd by more than 500 kHz, the input signal level W at which an AGC operation of the first AGC voltage starts is set lower than the input signal level N at which an AGC operation of the second AGC voltage starts. The frequency characteristics of the input signal levels at which an AGC operation starts correspond to the frequency characteristics of the inter-stage tuning circuit 21d and the intermediate frequency tuning circuit 24, respectively.
With the above measure, an AGC operation of the second AGC voltage is performed on an interference signal occurring in a strong-electric field area whose frequency is distant from Fd by about 100 kHz, whereby the gain of the radio-frequency circuit 21 is decreased and the degree of interference is lowered. The degree of interference can further be lowered by increasing the level of the second AGC voltage and thereby decreasing the gain of the radio-frequency circuit 21 further. However, this also suppresses the level of a broadcast signal to be received that is input to the mixing circuit 22, resulting in a sensitivity failure and deterioration in noise characteristic. The level of the second AGC voltage is set in view of a tradeoff between the interference and the reception sensitivity.
For a strong interference signal whose frequency is distant from Fd by more than several hundreds of kilohertz (e.g., 500 kHz), an AGC operation of the first AGC voltage is performed.
There may occur a case that another area where a strong-electric-field broadcast signal exists whose frequency is distant from Fd by 400 kHz exists adjacent to an area where a plurality of strong-electric-field broadcasting stations exist close to each other. In this case, an AGC operation of the second AGC voltage also functions to lower the reception sensitivity of a broadcast signal to be received.